[PATCH] Add pg_disable_checksums() and supporting infrastructure

Extracted from a larger patch, this patch provides the basic infrastructure for turning data
checksums off in a cluster. This also sets up the necessary pg_control fields to support the
necessary multiple states for handling the transitions.

We do a few things:

- Change "data_checksums" from a simple boolean to "data_checksum_state", an enum type for all of
the potentially-required states for this feature (as well as enabling).

- Add pg_control support for parsing/displaying the new setting.

- Distinguish between the possible checksum states and be specific about whether we care about
checksum read failures or write failures at all call sites, turning DataChecksumsEnabled() into two
functions: DataChecksumsEnabledReads() and DataChecksumsEnabledWrites().

- Create a superuser function pg_disable_checksums() to perform the actual disabling of this in the
cluster.

I have *not* changed the default in initdb to enable checksums, but this would be trivial.